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(11) | EP 1 790 758 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Atomic layer deposition (ald) method for producing a high quality layer |
| (57) The present invention is related to an ALD method comprising the steps of: a) providing a semiconductor substrate in a reactor, b) providing a pulse of a first precursor gas into the reactor at a first temperature, c) providing a first pulse of a second precursor gas into the reactor at a second temperature, d) providing a second pulse of the second precursor gas at a third temperature lower than the second temperature, and e) optionally repeating at least once step b) through step d) till a desired layer thickness is achieved. It is also related to a reactor suitable to apply the method. |
Field of invention
[0001] The present invention is related to an improved method for Atomic Layer deposition. Further, it is also related to a reactor design suitable for applying the method.
Background
[0002] Because in the field of electronics in general further downscaling of the semiconductor devices is always proceeding, deposition processes have to be developed able to deposit layers with a thickness control at atomic layer scale.
[0003] One of these deposition techniques is Atomic Layer Deposition (ALD), often used for depositing dielectric layers.
[0004] Atomic Layer Deposition is a thin film deposition technique based on the use of separated chemisorption reactions of at least two gas phase reactants with a substrate.
[0005] There are two characteristics which can limit the quality and scalability of layers deposited by ALD. A first characteristic is the growth-per-cycle (GPC), which often is much lower than the theoretical maximum of one monolayer per cycle. This can result in film roughness and slow film closure, which makes especially thin films (thinner than about 5 nm) prone to localized defects such as pinholes.
[0007] For example, in the deposition of hafnium oxide from hafnium tetrachloride and water, the growth-per-cycle is only 20% of a monolayer and the Cl-impurities remain in the deposited layer.
Aim of the invention
[0008] The present invention aims to provide an ALD method and reactor to fabricate a high-quality ALD layer under optimized process conditions.
Summary of the invention
[0009] The present invention provides an ALD method comprising the steps of:
- a) providing a semiconductor substrate in a reactor
- b) providing a pulse of a first precursor gas into the reactor at a first reactor temperature,
- c) providing a first pulse of a second precursor gas into the reactor at a second temperature,
- d) providing a second pulse of the second precursor gas at a third temperature lower than the second temperature, and
- e) optionally, repeating at least once step b) through step d) till a desired layer thickness is achieved.
[0010] The first precursor gas can be a halide or an oxyhalide such as POCl3, and more particularly it can be a metal halide or a metal oxyhalide such as HfCl4, TaCl5, WF6, WOCl4, ZrCl4, AlCl3, TiCl4, SiCl4 or the like.
[0011] The second precursor gas can be any precursor able to decompose the first precursor, or to eliminate the ligands of the first precursor. More particularly it can be H2O, H2O2, O2, O3, NH3, H2S, H2Se, PH3, AsH3, C2H4 or Si2H6.
[0012] In a method according to the invention said first temperature can be between about 100°C and about 800°C, preferably between about 150°C and about 650°C, or between about 200°C and about 500°C, or more preferably between about 225°C and about 375°C.
[0013] In a method according to the invention said second temperature can be substantially equal or higher than the first temperature, more particularly between about 100°C and about 800°C, preferably between about 150°C and about 650°C, or between about 200°C and about 500°C, or more preferably between about 225°C and about 375°C.
[0014] In a method according to the invention said third temperature can be substantially lower than the second temperature, preferably lower than about 500°C, or than about 350°C, or than about 225°C and more preferably is room temperature.
[0015] A method according to the invention can further comprise the step of heating the substrate surface at a fourth temperature, between step c) and d).
[0016] In a method according to the invention said fourth temperature can be substantially equal or higher than the second temperature, preferably higher than about 375°C and more preferably equal to about 500°C.
[0017] In a method according to the invention, the step of heating the substrate surface at a fourth temperature can be performed in inert atmosphere.
[0018] In a method according to the invention, the substrate can be exposed to a plasma treatment during and/or after step d).
[0019] The plasma used for said plasma treatment can consist of N2O, NO, O2, N2, H2, NO2, or NH3, etc.
[0020] In a preferred method according to the invention the first and second temperature is about 300°C, the third temperature is room temperature and the fourth temperature (if any) is about 500°C.
[0021] The present invention also provides an ALD reactor suitable for carrying out a method according to the invention.
[0022] Such an ALD reactor comprises:
- i. a first and a second susceptor,
- ii. means for heating a substrate when present on the first susceptor, and
- iii. means for cooling the substrate when present on the second susceptor.
[0023] The reactor according to the invention can further comprise means for transporting a semiconductor substrate between both susceptors.
Short description of the drawings
[0025] Figure 1: Effect of the H2O pulse length on the GPC as determined by RBS (Rutherford Backscattering Spectroscopy).
[0026] Figure 2: Effect of the H2O pulse length on the TOFSIMS (Time-Of-Flight Secondary Ion Mass Spectroscopy) Cl-profile.
[0027] Figure 3: Growth-per-cycle (RBS) as a function of the number of cycles for HfO2 on 1 nm chemical oxide substrates.
[0028] Figure 4: Decay of the TOFSIMS Si intensity as a function of the RBS Hf-coverage for HfO2 deposited with IRPA and standard ALD on chemical oxide substrates, and for standard HfO2 on different starting substrates.
[0029] Figure 5: TOFSIMS Cl-profiles of HfO2 deposited with different extended reaction cycles (a) and after different post deposition anneals (b).
[0030] Figure 6: TOFSIMS Cl-profiles of HfO2 deposited with intermediate cooling and longer H2O pulses.
Description of the invention
[0031] Atomic Layer Deposition (ALD) is a thin film deposition technique based on the use of separated chemisorption reactions of at least two gas phase reactants with a substrate. Such gas phase reactants are also called precursor gasses.
[0032] There are two characteristics which can limit the quality and scalability of layers deposited by ALD.
[0033] A first characteristic is the growth-per-cycle (or growth rate), which is often much lower than the theoretical maximum of one monolayer per cycle. This can result in film roughness and slow film closure, which makes especially thin films (on the order of less than 5 nm) prone to localized defects such as pinholes.
[0035] In order to optimize the above characteristics, the present invention provides an ALD method comprising the steps of:
- a) providing a semiconductor substrate in a reactor,
- b) providing a pulse of a first precursor gas into the reactor at a first temperature,
- c) providing a first pulse of a second precursor gas into the reactor at a second temperature,
- d) providing a second pulse of the second precursor gas at a third temperature lower than the second temperature, and
- e) optionally repeating at least once step b) through step d) till a desired layer thickness is achieved.
[0036] The semiconductor substrate can comprise or can consist of any semiconductor material(s) suitable in the field of IC processing. In particular it can comprise or can consist of silicon, germanium or silicon germanium.
[0037] The layer obtainable by a method according to the invention can be of a substantially pure element (e.g. Si, Cu, Ta, W), an oxide (e.g. SiO2, GeO2, HfO2, ZrO2, Ta2O5, TiO2, Al2O3, POx, VOx, CrOx, FeOx, ZnO, SnO2, Sb2O5, B2O3, In2O3, WO3), a nitride (e.g. Si3N4, TiN, TaNx, AlN, BN, GaN, NbN, MoxN, WxN), a carbide (e.g. SiC), a sulfide (e.g. CdS, ZnS, MnS, WS2, PbS), a selenide (e.g. CdSe, ZnSe), a phosphide (e.g. GaP, InP), an arsenide (e.g. GaAs, InAs), or mixtures thereof.
[0038] The first precursor gas can be a halide or an oxyhalide such as POCl3. More particularly it can be a metal halide or a metal oxyhalide. For example, said first precursor can be HfCl4, TaCl5, WF6, WOCl4, ZrCl4, AlCl3, , TiCl4, SiCl4 or the like.
[0039] The second precursor gas can be any precursor able to eliminate the ligands of the first precursor. More particularly it can be H2O, H2O2, O2, O3, NH3, H2S, H2Se, PH3, AsH3, C2H4 or Si2H6.
[0040] In the context of the invention the first, second and third temperature is to be understood as the temperature in the reactor, unless otherwise stated.
[0041] In a method according to the invention the first temperature can be between (about) 100°C and (about) 800°C, preferably between (about) 150°C and (about) 650°C, or between (about) 200°C and (about) 500°C, and more preferably between (about) 225°C and (about) 375°C.
[0042] In a method according to the invention the second temperature can be substantially equal or higher than the first temperature, between (about) 100°C and (about) 800°C, preferably between (about) 150°C and (about) 650°C, or between (about) 200°C and (about) 500°C, and more preferably between (about) 225°C and (about) 375°C.
[0043] In a method according to the invention the third temperature can be substantially lower than the second temperature, preferably lower than (about) 500°C, or lower than (about) 350°C, or lower than (about) 225°C and more preferably is room temperature.
[0044] A method according to the invention can further comprise the step of heating the substrate surface at a fourth temperature, between step c) and d).
[0045] In a method according to the invention this fourth temperature can be substantially equal or higher than the second temperature, higher than (about) 375°C and preferably equal to (about) 500°C.
[0046] Said step of heating the substrate surface at a fourth temperature can be performed in inert atmosphere, such as but not limited to nitrogen or argon.
[0047] In a method according to the invention, the substrate can be exposed to a plasma treatment during and/or after step d). The plasma can be direct or remote.
[0049] In a preferred method according to the invention the first and second temperature is (about) 300°C, the third temperature is room temperature and the fourth temperature is (about) 500°C.
[0050] The invention provides also a new reactor design that is suitable for performing the method of the invention and in which reactions can be performed at their optimized temperature.
[0051] Such an ALD reactor comprises:
- a first and a second susceptor,
- means for heating a substrate when present on the first susceptor, and
- means for cooling the substrate when present on the second susceptor.
[0052] In the framework of the invention, a susceptor is any means, e.g. a plate, suitable for supporting (bearing) the substrate upon which the layer is to be deposited.
[0053] The ALD reactor according to the invention can further comprise means for transporting a semiconductor substrate between both susceptors.
[0054] The ALD reactor according to the invention can further comprise means for producing a plasma.
[0055] According to the invention, the means for heating can comprise flash lamps or any means to create a temperature increase of at least the substrate surface.
[0056] According to the invention, the means for cooling can comprise a recirculating cooling medium, such as cooled nitrogen, or a Peltier element or any means to create a temperature decrease of at least the substrate surface.
[0057] In a particular embodiment according to the present invention, a hafnium oxide layer can be deposited using a method of the invention wherein hafnium tetrachloride is the first precursor gas and water the second precursor gas.
[0058] More particularly, an ALD method for depositing an hafnium oxide layer can comprise the steps of:
- i. providing a semiconductor substrate in the ALD reactor,
- ii. providing a pulse of HfCl4 in the reactor at 300°C,
- iii. providing a first pulse of H2O in the reactor at 300°C,
- iv. optionally heating the substrate at 500°C in N2 atmosphere,
- v. providing a second pulse of H2O in the reactor at room temperature, while the substrate can be exposed to an N2O plasma treatment, and
- vi. optionally, repeating at least once step ii) through step v).
[0059] The reaction cycle (step (ii) through step (v)) can then be repeated until the desired layer thickness is obtained.
[0060] The step of providing the second pulse of H2O can also comprise exposure to moisture present in the reactor.
[0061] The steps of providing a pulse of HfCl4, providing a first pulse of H2O and heating the substrate can be performed on the first susceptor. The temperature is maintained constant at (about) 300°C. Heating means, e.g. (flash) lamps, are provided for fast wafer surface temperature increase from (about) 300°C to (about) 500°C when the substrate is present on the first susceptor.
[0063] The second H2O pulse (which can comprise moisture exposure during transportation in this case) and optionally plasma exposure can be performed on the second susceptor at lower temperature. Cooling means are provided for wafer surface temperature decrease below (about) 225°C or even to room temperature.
[0064] By means of conventional ALD, the deposition of hafnium oxide from hafnium tetrachloride and water, the growth-per-cycle is only 20% of a monolayer and the Cl-impurities remain in the deposited layer.
[0065] By means of the method according to the invention, the growth-per-cycle can be enhanced from 15% to 40% of a monolayer and the Cl-content can be reduced by 2 orders of magnitude.
[0066] Though the following example describes only the deposition of HfO2 from HfCl4 and H2O, it is to be understood that said first and second precursors as defined in the present invention are expected to follow the same chemical mechanisms as described in the example section, when carrying out a method of the invention. Therefore, the present invention is not intended to be limited to the following illustrative example.
Example
[0067] Atomic Layer Deposition (ALD) is based on the use of separated chemisorption reactions of at least two gas phase reactants with a substrate.
[0068] For the deposition of HfO2 from HfCl4 and H2O, HfCl4, which is the first precursor, chemisorbs on the substrate by reaction with surface -OH groups:
x OH* + HfCl4 → OxHfCl4-x* + x HCl
[0069] The chemisorption must be self-limiting and saturated within the time of the precursor pulse. When full saturation is reached, the excess of HfCl4 and the gas reaction byproducts are purged away by an nitrogen flow, further referred to as a nitrogen purge. Thereafter, pulse and nitrogen purge steps are repeated with the H2O precursor in order to hydrolyze the Hf-Cl bonds.
OxHfCl4-x* + (4-x) H2O → OxHf(OH)4-x + (4-x) HCl
[0071] Usually, the growth-per-cycle is less than 1 monolayer because of the following limitations: either the number of reactive sites on the substrate is limited, or steric effects from bulky precursors limit the amount of material to be chemisorbed.
[0072] Moreover, the growth-per-cycle can depend on the starting substrate - when the substrate changes from the starting substrate (for example silicon substrate) to the deposited material itself. When starting substrate effects have vanished, the growth-per-cycle becomes constant and is referred to as the steady growth-per-cycle.
[0074] For the conventional HfCl4/H2O process (also referred to as Standard HfCl4/H2O ALD process), the steady growth-per-cycle at 300°C is only 15% - 17 % of a monolayer (also referred to as oML). Several experimental observations indicate that the low HfO2 growth-per-cycle is caused by limitation of the number of reactive sites, the -OH groups in the HfCl4 reaction.
[0075] For example, the decreasing growth-per-cycle as a function of temperature is attributed to decreased hydroxylation of the HfO2 surface. This implies that the HfCl4 reaction with oxygen bridging sites is not very efficient. Indeed, it has been shown that the ZrCl4 species (the HfCl4 and ZrCl4 chemistries are very similar) are not very reactive towards siloxane bridges, since the preheating temperature of the support influences the amount of adsorbed Zr.
[0076] Furthermore, substrate inhibition occurs typically on surfaces with too low -OH density, for example HF cleaned Si.
[0077] As an illustration of several embodiments of the present invention, an ALD reaction cycle with different intermediate treatments, such as exposure to gas phase moisture, plasma treatments or thermal anneals is studied and compared to the Standard ALD cycle.
[0078] In the case of an ALD cycle with plasma treatments, which is a particular embodiment of the present invention, this extended ALD process is also referred to as Intermediate Remote Plasma Assisted (IRPA) ALD.
[0079] In the case of an ALD cycle with a thermal anneal, which is also a particular embodiment of the present invention, this extended ALD process is also referred to as Intermediate Thermal Anneal (ITA) ALD.
[0080] A first goal is to increase the number of -OH groups at the HfO2 surface, in order to enhance the growth-per-cycle. A theoretical model predicts that the higher the growth-per-cycle, the lower the number of cycles required for film closure and the lower the roughness of the film. Therefore, the improvement of the quality and scalability of HfO2 layers by enhancing the growth-per-cycle with intermediate treatments is investigated.
[0081] A second goal is to reduce the Cl-content of the HfO2 layers. Indeed, the intermediate treatments can affect the growth-per-cycle and the Cl-content.
[0082] All samples are processed in a Polygon 8200 cluster. Prior to deposition, a chemical oxide of 1 nm thickness is grown. HfO2 is deposited in an ALDTM Pulsar 2000 reactor, a hot wall cross-flow type reactor with inert gas valving. The pressure in the reactor is 1.33 mbar (1 Torr). All pulses with HfCl4 precursor and first pulses with H2O precursor are performed at 300°C. HfCl4 is a solid at room temperature. It is heated to approximately 185°C to achieve sufficient vapor pressure for the HfCl4 pulses. The pulse and inert gas purge times for the Standard process are indicated in Table 1. Elongation of the purge times after HfCl4 or H2O pulses up to 5 minutes does not change the growth-per-cycle, suggesting that the precursor pulses are well separated in the standard conditions. Moreover, this indicates that there is sufficient recovery time of the HfCl4 solid source between different pulses.
Table 1: Pulse and inert gas purge parameters in the standard HfO2 deposition.
| HfCl4 pulse (s) | 0.1 |
| N2 purge (s) | 1 |
| H2O pulse (s) | 0.3 |
| N2 purge (s) | 3 |
[0083] Samples are measured by spectroscopic ellipsometry (SE) on a KLA-Tencor ASET F5. RBS is performed in a RBS400 end station (Charles Evans and Associates) with a 1 MeV He+ beam. TOFSIMS depth profiles are measured with an IonTOF-IV instrument using a dual beam set-up with a 500 eV Ar+ ion beam.
[0084] Intermediate treatments are performed in an Epsilon Nitride CVD (Chemical Vapor Deposition) reactor, equipped with a remote Microwave Radical Generator (MRG) or in the transport module of the polygon cluster.
[0085] The steady growth-per-cycle for the Standard HfCl4/H2O ALD process at 300°C (as described in Table 2) is 1.4 Hf/nm2 or 15% of a monolayer (figure 1), in agreement with literature. The growth-per-cycle is not fully saturated with the standard H2O pulse of 0.3 seconds, as shown in figure 1. The growth-per-cycle saturates at a H2O pulse of about 10 seconds to a value of 20% of a monolayer. Also the Cl-content is saturated at 10 seconds, as shown by TOFSIMS (figure 2). The growth-per-cycle does not depend on the HfCl4 pulse length.
[0086] The effect of the 1 nm chemical oxide substrate on the growth-per-cycle for Standard HfCl4/H2O ALD compared to IRPA ALD is shown in figure 3. In the first reaction cycle, the growth-per-cycle is almost 3 times higher than the steady growth-per-cycle (4.3 Hf/nm2 or 47% of a monolayer) in agreement with literature. The substrate enhancement only acts in the first reaction cycle. This is surprising, as the chemical oxide substrate can by no means be fully covered by HfO2 in the first cycle, as less than half a ML is deposited. Therefore, one would expect to see growth enhancement also in the second reaction cycle.
[0087] One possible explanation is that all Si-OH groups on the chemical oxide are consumed by reaction with HfCl4 in the first reaction cycle. Any Si-O-Si bridges left uncovered by HfO2 are not hydrolyzed in the H2O pulse of the first reaction cycle. Therefore, HfO2 in the second reaction cycle only reacts with the Hf-OH groups and the steady HfO2 growth-per-cycle is immediately reached. This would imply that, as the Si-O-Si sites are not reactive towards both HfCl4 and H2O precursors, no further growth is possible from the substrate; only sideward growth from HfO2 islands is possible to cover that area.
[0088] Another possibility is that both Si-OH and Si-O-Si bridges react with HfCl4. The reaction with the latter surface sites results in contamination of the Si substrate by Cl:
Si-O-Si* + HfCl4 → Si-O-HfCl3* + Si-Cl
[0089] Si-Cl bonds are difficult to hydrolyze at 300°C. Therefore, in the next reaction cycles HfO2 deposition on the Si-Cl sites is blocked. This would explain the larger Cl-content at the bottom interface of HfO2 layers. The fact that the HfO2 growth-per-cycle remains constant in the subsequent reaction cycles, and the decrease in Cl-content could indicate that nucleation on the HfO2 substrate proceeds only on the Hf-OH groups, and that Hf-O-Hf bridges are less reactive than Si-O-Si bridges.
[0090] Plasma treatment every cycle enhances the steady growth-per-cycle from 15% of a monolayer to 42% of a monolayer (figure 3 and table 2).
[0091] A theoretical model for random deposition in ALD predicts that the higher the growth-per-cycle, the smaller the number of cycles required for film closure and the lower the roughness of the film. The growth mode of HfO2 layers with different growth-per-cycle can be investigated by TOFSIMS surface measurements. The decay of the Si substrate intensity is plotted as a function of the Hf-coverage from RBS (figure 4).
[0092] Indeed, a faster decay of the TOFSIMS Si intensity for IRPA ALD HfO2 as compared to Standard ALD HfO2 (figure 4). The difference with Standard HfO2 ALD is small but systematic.
[0093] In table 2 the GPC (% ML) for different extended reaction cycles is shown. The Hf-coverage is measured by RBS or SE on samples with 10 reaction cycles. The growth-per-cycle is calculated as the average over the last 9 reaction cycles to exclude the enhancement effect of the substrate in the first reaction cycle.
Table 2: Growth-per-cycle for different reaction cycles.
| Description of the reaction cycle | GPC RBS (Hf/nm2) | GPC RBS (% ML) | GPC SE (% ML) | |
| 1 | Standard | 1.40-1.50 | 15 - 16 % | 18 - 20 % |
| 2 | Standard + RT plasma treatment | 4.10 | 42 % | 49 % |
| 3 | Standard + 5 min cooling | 3.21 | 34 % | 40 % |
| 4 | Standard + 90 sec cooling | 2.89 | 32 % | 35 % |
| 5 | Standard + 2 min 420°C + 2 min cooling | 2.95 | 32 % | 35 % |
| 6 | HfCl4 pulse + 4 min cooling | 0.85 | 9 % | 12 % |
| 7 | HfCl4 pulse + 2 min 420°C + 4 min cooling | 0.88 | 10 % | 11 % |
[0094] Growth enhancement is also observed for room temperature treatments without plasma (table 2 reaction cycle 3 and 4). The growth-per-cycle (34% of a monolayer) is slightly lower as compared to the RT (room temperature) remote plasma treatments (table 2 reaction cycle 2) (42% of a monolayer). Thus, an important contribution of the growth enhancement during plasma treatments comes from lowering the temperature. Therefore, one could suggest that the enhanced growth is caused by H2O adsorption when the wafer is cooled down to room temperature:
Hf-O-Hf* + H2O → 2 Hf-OH*
[0095] It has been shown that for moisture concentrations as low as 10-100 ppm are sufficient for monolayer coverage of H2O on HfO2 at room temperature. Thus, the moisture background in the transport module could suffice for the introduction of -OH groups during the transport from ALD to nitride reactor and back. The adsorption of H2O in the transport module saturates within 90 seconds: similar growth enhancement is achieved for 90 seconds or 5 minutes intermediate cooling.
[0096] Support for the H2O reaction in the transport module comes from the observation that HfO2 can be deposited without H2O pulse in the ALD reactor at 300°C, but with intermediate cooling in the transport module instead (Table 2 reaction cycle 6 and 7). Probably, the Cl-content of this HfO2 layer is very high, as the Cl removal becomes more difficult at lower temperatures. The expected high Cl-content might explain why the growth-per-cycle for this process is slightly lower than for the standard ALD process.
[0097] The growth-per-cycle is also enhanced for intermediate anneal at 420°C in the nitride reactor (table 2 reaction cycle 5), with the same amount as for intermediate cooling (table 2 reaction cycle 3 and 4). One would expect that the -OH density on the HfO2 substrate decreases by thermal treatment, and as such, the growth-per-cycle is decreased. This probably indeed occurs during the intermediate anneal. However, the intermediate anneal is performed in the nitride reactor on the polygon platform, and the transport from nitride to ALD reactor takes about 2 minutes. As shown above, the HfO2 surface can easily re-adsorb moisture during the cooling that occurs during this transport.
[0098] Analysis of the Cl content in HfO2 layer can give more information on the ALD reaction mechanism because it shows the efficiency of Cl-removal by the H2O reaction and/or the intermediate treatments applied after the H2O reaction. TOFSIMS Cl-profiles of standard HfO2 are therefore compared with HfO2 deposited with the different extended reaction cycles (figure 5 and figure 6). The intermediate treatments are applied only every 10 reaction cycles instead of after every single reaction cycle in order to speed up the deposition of 4 nm HfO2 layers (minimum thickness to obtain a reasonable Cl-profile). The effect of the intermediate treatment on the Cl content is still apparent in the Cl-profiles (figure 5 and figure 6).
[0099] One can see that HfO2 deposited with plasma treatments or cooling in moisture contains about twice as much Cl as the Standard HfO2 process (figure 5 and figure 6). Apparently, the standard H2O pulse of 0.3 seconds is too short to hydrolyze the much larger amount of Hf-Cl bonds, introduced by the enhanced growth-per-cycle. Therefore, the H2O pulse time is re-optimized for the enlarged growth-per-cycle. H2O pulses of 10 seconds are sufficient to reduce the Cl-content too a similar level as for the process without intermediate cooling (Figure 6).
[0100] Cl can efficiently be removed by intermediate thermal treatments (figure 5 (a)). The temperature and frequency of the intermediate anneal determines the efficiency of the Cl-elimination. A reduction of 2 orders of magnitude is obtained with intermediate anneals at 500°C every 10 cycles. On the other hand, the Cl-content is not reduced during a post deposition anneal at 500°C (Figure 5 (b)). For intermediate thermal anneals at 420°C every 10 cycles, the Cl-content is reduced as compared to intermediate cooling, but the Cl-level is still at the same level as for the standard HfO2 process. Applying a longer H2O pulse gives in this case a larger improvement. The ambient of the intermediate anneal (O2 or N2) does not influence the Cl-content of the layers. However, it can have a significant impact on the thickness of the interfacial layer. For O2 anneals at 500°C, the interfacial oxide easily grows to more than 1 nm.
[0102] Having regard to HfCl4 reaction, in agreement with literature, it is proposed that HfCl4 reacts with -OH groups and not with Hf-O-Hf bridges:
x OH* + HfCl4 → OxHfCl4-x* + x HCl
[0103] All experimental observations in this work support that in the standard process, the amount of Hf deposited in this half reaction is controlled by the number of -OH groups on the substrate, and not by steric hindrance of the -Cl ligands.
[0104] This is further investigated by using a recent model of growth-per-cycle (Table 3). The model is based on the mass balance of chemisorption and assumes a two-dimensional arrangement of the adsorbed ligands.
[0105] For the standard HfO2 process, the growth-per-cycle is 1.4 Hf/nm2 for plasma treatments [Table 2]. During the HfCl4 reaction, each Hf atom brings along four Cl ligands (using the HfCl4 precursor). Thus, according to mass balance, about 5.6 (= 4.1 x 4) Cl ligands /nm2 arrive to the surface. The maximum number of Cl ligands remaining on a flat surface when steric hindrance prevails can be estimated from the van der Waals radius of Cl (0.175 nm as 9.4 /nm2). Thus, for the standard process, the amount of Cl after HfCl4 reaction is still lower than the maximum allowed by steric hindrance. However, the maximum amount of Cl is most likely slightly lower than this theoretical value because this upper limit does not consider the specific bond arrangements in the Hf-Cl layer. Thus for the standard process, the model cannot give much information about the OH group content of the substrate.
[0106] On the other hand, the model can give some information in case of growth enhancement with intermediate treatments. The growth-per-cycle is 4.1 Hf/nm2 for plasma treatments [Table 2]. Thus, according to mass balance, about 16.4 (= 4.1 x 4) Cl ligands /nm2 arrive to the surface. This is much higher than the maximum number of Cl ligands allowed on a flat surface (9 /nm2). Therefore, at least (16.6 - 9.4) ≈ 7 Cl /nm2 have been removed by reaction with -OH (Table 3). Thus, this number also is a rough estimation of the -OH group density on HfO2 after plasma treatment. For intermediate cooling, the -OH group density calculated in a similar way is 2-3 /nm2 (Table 3).
Table 3: Amount of -OH groups on HfO2 for different intermediate treatments after the H2O ALD half reaction obtained by using the steric hindrance model.
| Description of the extended reaction cycle | # OH (/nm2) |
| Standard + RT plasma treatment | 7 |
| Standard + 5 min cooling | 3 |
| Standard + 90 sec cooling | 2 |
| Standard + 2 min 420°C + 2 min cooling | 2 |
[0107] Having regard to H2O reaction, the HfCl bonds introduced during the HfCl4 pulse are hydrolyzed in the H2O reaction:
HfClx* + x H2O → Hf(OH)x* + x HCl
[0108] At 300°C, this reaction is not complete as some residual Cl is present in the HfO2 layer even if the H2O pulse is saturated.
[0109] As the temperature of the H2O reaction decreases, dissociation of H2O on oxygen bridges also becomes important:
Hf-O-Hf* + H2O → 2 Hf-OH*
[0110] An effective way of eliminating Cl is by intermediate thermal annealing after the H2O pulse. The Cl-content is independent of the anneal ambient (O2 or N2). It is proposed that neighboring Hf-Cl and Hf-OH groups at the top surface react and release HCl:
HfCl* + HfOH → HfOHf* + HCl
[0111] As such, oxygen bridges are created. It is proposed that this reaction becomes more important as the temperature of the anneal increases.
[0112] Thus, in order to enhance the growth-per-cycle from 15% to 40% of a monolayer and to reduce the Cl-content by 2 orders of magnitude, the following extended HfCl4/H2O ALD reaction cycle can be proposed:
- HfCl4 reaction at 300°C
- H2O reaction at 300°
- Annealing in N2 for Cl-elimination at 500°C
- Hydroxylation with N2O plasma during and/or after moisture exposure at low temperature.
1. An Atomic Layer Deposition method comprising the steps of:
a) providing a semiconductor substrate in a reactor,
b) providing a pulse of a first precursor gas into the reactor at a first temperature,
c) providing a first pulse of a second precursor gas into the reactor at a second temperature,
d) providing a second pulse of the second precursor gas at a third temperature lower than the second temperature, and
e) optionally, repeating at least once step b) through step d) till a desired layer thickness is achieved.
2. A method according to claim 1, wherein the semiconductor substrate comprises any semiconductor
material suitable in the field of IC processing.
3. A method according to claim 2, wherein said semiconductor substrate comprises silicon,
germanium or silicon germanium.
4. A method according to any of claims 1 to 3, wherein said first precursor gas is a
halide or an oxyhalide.
5. A method according to claim 4, wherein said halide is a metal halide or said oxyhalide
is a metal oxhyhalide.
6. A method according to claim 4 or 5 wherein said first precursor gas is HfCl4, TaCl5, WF6, WOCl4, ZrCl4, AlCl3, POCl3, TiCl4, or SiCl4.
7. A method according to any of claims 1 to 6, wherein said second precursor gas is any
precursor able to eliminate the ligands of the first precursor.
8. A method according to claim 7 wherein said second precursor gas consists of H2O, H2O2, O2, O3, NH3, H2S, H2Se, PH3, AsH3, C2H4 or Si2H6.
9. A method according to any of claims 1 to 8, wherein said first temperature is comprised
between 100°C and 800°C.
10. A method according to any of claims 1 to 9, wherein said second temperature is substantially
equal or higher than said first temperature.
11. A method according to any of claims 1 to 10 further comprising the step of heating
the substrate surface at a fourth temperature, between step c) and d).
12. A method according to claim 11, wherein said fourth temperature is substantially equal
or higher than the second temperature.
13. A method according to claim 11 or 12, wherein the step of heating the substrate surface
at a fourth temperature is performed in inert atmosphere.
14. A method according to any of claims 1 to 13, wherein said first temperature and said
second temperature are 300°C, and said third temperature is room temperature.
15. A method according to claim 14 wherein, said fourth temperature is 500°C.
16. A method according to any of claims 1 to 15, wherein said substrate is exposed to
a plasma treatment during and/or after step d).
17. A method according to claim 16, wherein said plasma consists of N2O, NO, O2, N2, H2, NO2 or NH3.
18. A method according to any of claims 1 to 17, wherein said first precursor gas is hafnium
tetrachloride and said second precursor gas is H2O.
19. A method according to claim 18, wherein the substrate is exposed to an N2O plasma treatment during and/or after step d).
20. An Atomic Layer Deposition reactor comprising:
- a first and a second susceptor,
- means for heating a substrate when present on the first susceptor, and
- means for cooling the substrate when present on the second susceptor.
21. A reactor according to claim 20 further comprising means for transporting a semiconductor
substrate between both susceptors.
22. A reactor according to claim 20 or 21 further comprising means for producing a plasma.
23. A reactor according to any of claims 20 to 22, wherein said means for heating comprise
any means to create a temperature increase of at least the substrate surface.
24. A reactor according to claim 23, wherein said means for heating are flash lamps.
25. A reactor according to any of claims 20 to 24, wherein said means for cooling comprise
any means to create a temperature decrease of at least the substrate surface.
26. A reactor according to claim 25, wherein said means for cooling is a recirculating
cooling medium or a Peltier element.